Question

[Quillen]

Tính nhanh 

\(A=\frac{\frac{1}{2}+\frac{1}{3}+....+\frac{1}{2010}}{\frac{2009}{1}+\frac{2008}{2}+...+\frac{1}{2009}}\)

Answer 1

[ʚɞＯＮＬＹღＹＯＵ╰❥]

Gọi \(S=\frac{2009}{1}+\frac{2008}{2}+...+\frac{1}{2009}\)

\(\Rightarrow S=\frac{2010-1}{1}+\frac{2010-2}{2}+...+\frac{2010-2009}{2009}\)

\(\Rightarrow S=2010-1+\frac{2010}{2}-1+...+\frac{2010}{2009}-1\)

\(\Rightarrow S=2010+\frac{2010}{2}+...+\frac{2010}{2009}-\left(1+1+..+1\right)\)

\(\Rightarrow S=2010+\frac{2010}{2}+...+\frac{2010}{2009}-2009\)

\(\Rightarrow S=\frac{2010}{2}+\frac{2010}{3}+...+\frac{2010}{2009}+1\)

\(\Rightarrow S=\frac{2010}{2}+\frac{2010}{3}+..+\frac{2010}{2009}+\frac{2010}{2010}\)

\(\Rightarrow S=2010\left(\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2010}\right)\)

Khi đó \(A=\frac{\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2010}}{2010\left(\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2010}\right)}=\frac{1}{2010}\)